Function push_rect_quad 

pub fn push_rect_quad(
    out: &mut Vec<Vertex>,
    x: f32,
    y: f32,
    w: f32,
    h: f32,
    color: Color,
)

Append six vertices (two triangles) covering the axis-aligned rectangle (x, y, w, h) with a uniform color, tagged as a solid rectangle.